An apparatus of the kind mentioned above is described in the article "Radio Test performance of a Narrowband TDMA system" by Stjernvall, Hedberg and Ekemark, published in IEEE Vehicular Tech. Conf. Tampa. Florida, U.S.A., June 1987. A similar apparatus is also described below with reference to FIG. 1.
The frequency error in a receiver with at least a roughly adjusted frequency is estimated in a unit for calculating frequency error, and a control processor generates a correction signal for each new time slot in response to the magnitude of the correction during the previous time slot and the estimated frequency error. The correction signal is supplied to a controllable local oscillator, which is a frequency synthesizer, according to the mentioned article, and is adapted to generate mixing signals, the frequencies of which are corrected in response to the control processor signal. The latter is digital and in practice is converted to analog form in a digital-analog converter before being supplied to the local oscillator. The number of steps with which the frequency from the oscillator can be adjusted is therefore dependent on the number of binary bits which the converter can convert. If a relatively small and simple converter is used the result will be that either the maximum frequency from the local oscillator is relatively small, or the frequency steps are relatively large.
Particularly with a coherent receiver, it is of great importance that the local frequency generator is locked to the transmitter frequency with rather good accuracy. This is because such a receiver may be said to operate with two dimensions, taking into account both phase and amplitude of the receiver signal.
From U.S. Pat. No. 4,520,493 it is known to perform frequency correction in the baseband of a signal received in a receiver. Only relatively small frequency errors can be corrected, however. For a large frequency error in a local oscillator in the receiver parts of the frequency band of the received signal will namely come outside the passband of the receiver filter.